It is difficult for nanosized ceramic powder of a particle diameter of 100-500 nm to be well distributed and appropriately sintered, because a conventional solid-state reaction route is much constrained by the quality of starting materials. Agglomeration or uneven distribution of particle diameters of starting nanosized powder materials necessitates additional processing steps which precede whatever starting material-based steps. Furthermore, due to low diffusion coefficients of solid-state substances, nanosized powder produced by the solid-state reaction route cannot exist in a pure phase unless it is processed at a relatively high heat treatment temperature. However, when carried out at a high temperature for a long period of time, heat treatment leads to vermicular aggregation among crystallites—an intractable problem that confronts nanosized ceramic powder preparation processes nowadays. For instance, pure-phase YAG particles in nanosized powder can be synthesized by a conventional solid-state reaction route only at a relatively high heat treatment temperature (>1600° C.) and, unfavorably, on condition that the YAG particle diameters are often larger than 1 μm.
YAG nanosized powder can also be produced by a sol-gel process and, favorably, at a relatively low temperature, say less than 700° C., directly from an amorphous substance through crystallization. However, the sol-gel process requires a subsequent heat treatment process. Likewise, YAG nanosized powder can also be produced by a hydrothermal method which, apart from the aforesaid heat treatment process, requires a high pressure process and thereby is restrained by supercritical conditions of water.
Chemical coprecipitation, which is often used to synthesize YAG powder, entails coprecipitating highly soluble Y and Al, such as YNO3 and AlNO3, with a precipitant to produce a solid-state precipitate, and then the solid-state precipitate undergoes a heat treatment process to produce aYAG starting powder. Advantages of chemical coprecipitation include: Y, Al and Nd ions are uniformly distributed to an atomic level, YAG phase structure is directly formed in an amorphous state, usually greatly decreasing the temperature required for crystallization, and precluding a transition phase, such as the formation of YAP (YAlO3) or YAM (Y4Al2O9), but its YAG powder particle diameter is small and thus have to undergo a calcination process to allow the crystal to grow from 50 nm to 220 nm. However, aggregation among the crystals increases with the calcination temperature. Hence, the sintering density is compromised.
Hence, manufacturers nowadays deem it important to provide a method of inhibiting an irregular aggregation of a nanosized powder to thereafter process nanosized powders (starting materials) produced by different process techniques so as to enhance process efficiency and nanosized powder crystal quality, and avoid crystalline irregularity and abnormal growth, such as overlapping and prepare a nanosized powder that features reduced particle aggregation and satisfactory distribution.